Image capture devices may be categorized as capturing images by either a global shutter or capture technique or a scanning technique. A global capture technique, such as performed by a charge-coupled device (CCD), captures an entire image sensing array of data from an image at the same time. To capture an image or image data means to receive light and store image data representing the light received. While global capture techniques avoid motion distortion artifacts, one disadvantage is a light gathering pixel is required to hold onto its value until it has been readout which can result in an increased refresh period and therefore, a decreased refresh rate compared with a sensor using a scanning technique. Furthermore, increased semiconductor area for a second charge storage element is needed for each pixel so that the pixel charge can be offloaded as soon as possible. The latter is the principle used by an interline CCD sensor which means it has a roughly double area penalty.
Some image capture devices use scanning sensors. One example of a scanning sensor is a typical complementary metal oxide semiconductor (CMOS) sensor device found in consumer electronics like video cameras. The CMOS sensor uses a rolling shutter or capture technique which captures different portions of a frame of image data at different times. An interline CMOS sensor includes parallel lines of light sensitive pixels and their accompanying storage circuitry. The exposures for successive lines can have significant overlap. For example, with a 30 Hz refresh rate, there can be a 30 ms exposure window for each line resulting in just about all lines being actively exposed at the same time in the same frame. However, the readout of the lines is serialized or rolling (i.e. one at a time). While a line is being readout, adjacent lines to be read out later are still capturing, meaning receiving and storing light, of a scene that is changing over time so spatial and temporal discontinuities result.
In another example of a scanning sensor, an external shutter can be used with a CMOS sensor, and the shutter can control exposure of each line of pixels for capture of image data in succession in a direction of traversal during a frame while blocking the other lines. Thus, the lines capture different portions of the image frame at different points in time, in effect “rolling” through the frame. A rolling shutter or rolling capture technique provides an advantage in terms of refresh rate in that readout is continuous and semiconductor area can be smaller as additional storage pixels are not needed.
Other examples of scanning sensors include image sensors as used in analog Cathode Ray Tube (CRT) systems, laser scanners and other systems where a beam activates a light sensitive area of a sensor in a sequential manner. The image sensors are scanned in a scanning pattern. An example of a scanning pattern is a raster scan order or a raster order. For example, an analog cathode ray tube (CRT) has an image sensor across which a beam moves in horizontal lines, and the image sensor generates analog signals representing the image. In digital systems using raster scanning technology, the image sensor can include pixels activated in a scanning pattern.
As mentioned above the time differences of the captures in the scanning or rolling techniques introduce image artifacts, particularly when the image subject is moving. Differences in lighting, skew, and wobble are all known artifacts of the time differences of capture during the frame.
Multiple image capture devices with image sensors or an array of image sensors can be used to capture a scene, usually to provide a larger total field of view or to improve total resolution or refresh rate by having each sensor in the area focus on a smaller field of view. The field of views (FOV) of at least two image sensors can overlap. For example, a first FOV of sensor can have its bottom border overlap the top of a second FOV of the other sensor. Images from the two sensors may be stitched together to produce a composite image. However, when the sensors are operate by a scanning technique such as rolling capture (vertical succession in this example), the first and second sensors start scanning their respective images top down at the same time. Thus, the sensor focused on a bottom portion of a scene captures the rows in the overlapping border area at the start of a frame, but the sensor focused on the top portion of the scene captures the rows in the overlapping border area at the end of a frame. Where there are possibly moving images in the frame, artifacts further complicate stitching together of the areas of overlap in the image data from the first and second image sensors.